In calibrating instruments that are used to measure the volume and the concentrations of various gases in breath, it has long been the practice to supply the instruments with a pulsatile flow of calibration gas (usually air) from a device known as a calibration syringe. This syringe typically includes a piston and cylinder arrangement which pumps gas to the instrument, through a check valve, as the operator moves a handle that is connected to the piston between first and second positions. Because the cylinder ordinarily has a volume comparable to the volume of gas that is exhaled during a typical human breath, and because the ejection stroke of the piston takes about the same time as an exhaled breath, the use of the calibration syringe allows the instrument to be calibrated under conditions that simulate the conditions under which it will be used with test subjects.
The use of syringes of the above-described type has been found to result in sizable errors in the volume of gas supplied during calibration. One of these errors, known as "siphoning", results from the fact that the inertia of the gas flowing through the check valve has a tendency to keep that check valve open even after the piston has reached the end of its stroke. Such siphoning affects the accuracy of the calibration process by causing the actual volume of gas supplied to the instrument to be larger than might to be expected on the basis of the volume of the calibration syringe.
Another type of error, known as stroke speed error, is caused by the fact that the number of pulses produced by a gas turbine that monitors the flow of gas delivered by the syringe can vary over wide limits depending upon how fast the operator pushes the piston during its ejection stroke. As a result of this error, the apparent volume of gas delivered by the syringe during calibration (as measured by the turbine) can differ substantially from the actual volume of gas delivered, thereby affecting the accuracy of the volume calibration. The latter errors, in turn, introduce errors into all later measurements that are based on the inaccurate calibration.
Still another type of error that is associated with the use of manually operated calibration syringes is caused by the fact that, due to operator inattention, the piston may not be moved between exactly the same beginning and end positions during each ejection stroke. An operator may not, for example, withdraw the piston to its true outermost position, or may not push the piston to its true innermost position. Any such deviations from the desired inner and outer positions, of course, affect the volume of gas delivered by the syringe during calibration. These errors, in turn, affect the accuracy of all measurements that are based on the inaccurate calibration.